This invention relates to rotating field machines in general and more particularly to an improved method and apparatus for operating a converter fed three phase rotating field machine having a neutral return.
The control characteristics of three phase drives can be substantially improved if they are designed as converter fed rotating field machines as taught in Siemens-Zeitschrift 1971, pp. 174 to 176. As disclosed therein and as is common in the art the windings are fed with symmetrical sinusoidal phase currents much in the manner that rotating field machines fed directly from a three phase system are fed. In order to carry out such a feeding of phase currents the converter rectifiers used in a converter feeding such a machine must be selected to withstand the peak values of current and voltage. In this type of operation where sinusoidal phase currents are fed to the machine one rectifier must carry half wave peak of a phase current at points in time where the two other rectifiers have currents therethrough only half the maximum current. However, all of the rectifiers must be capable of carrying the peak value of the phase currents, even though this value is present only for short times. As a result the overall degree of utilization of the rectifiers is poor.
In view of this, it is the object of the present invention to achieve better utilization of the converter rectifiers in such a converter fed rotating machine without degrading the power and torque of the machine.